1. Field of the Invention
The present invention relates to a method for estimating shear wave reflection data and, more particularly, to a method for producing pseudoshear wave reflection seismic sections from compressional wave reflection data. This invention further relates to a method for determining the presence of hydrocarbons in subsurface geological formations by comparative assessment of compressional and shear wave reflection data.
2. Discussion of the Prior Art
The use of compressional, or P, wave reflection data in geophysics analysis is notoriously well known. Typically, seismic sections are produced from compressional wave subsurface reflections which provide extremely useful information concerning formation characteristics. Recently, interest has been growing in obtaining shear, or S, wave seismic sections which would provide additional useful information concerning subsurface formation characteristics which can be used in conjunction with information obtained from compressional wave seismic sections. For example, compressional wave seismic sections can provide useful information on the compressibility of subsurface formations, while shear wave seismic sections can provide useful information on subsurface formation rigidity. Shear wave seismic reflections are noisier than compressional wave seismic reflections, however, making interpretation difficult. Furthermore, the detection of shear wave reflections is more difficult than is the case with compressional wave reflections. The shear wave reflections are typically of much lower amplitude than compressional wave reflections, making detection difficult, and the direct propagation of a shear wave into a subsurface formation to induce a shear wave reflection require special transducers and additional steps over and above those required for obtaining compressional wave reflection data. This makes obtaining shear wave data difficult, costly and time-consuming.
It has long been recognized that compressional wave reflectivity is a function of incident angle. This behavior is described by the Zoeppritz equations. The Zoeppritz equations include a shear wave velocity information. While shear wave data may be estimated from conventional data by extracting such shear wave data from amplitude-offset data, in order to obtain a valid estimate of the shear reflectivity from real seismic data, it is necessary to consider a variety of other effects. For example, spherical spreading, attenuation, arrays and directivity all cause amplitude to vary with offset. Small errors in move-out velocity and statics can completely overwhelm the amplitude-offset signal.
While difficult to obtain, shear wave data can be very useful in the exploration for hydrocarbons. Hydrocarbon deposits which produce compressional wave reflection amplitude anomalies do not produce similar shear wave reflection amplitude anomalies. Such a result occurs because shear waves do not respond to any fluids and therefore do not produce different amplitude responses for gas, oil, and water. In contrast, compressional wave amplitude anomalies that are caused by anomalous lithologies such as salt, coal and hard streaks usually have equally anomalous shear wave behavior. Such an application of shear wave information has not been widely exploited, however, because most amplitude anomalies of interest occur offshore while shear wave seismic data can only be recorded onshore.
Efforts to apply shear wave information to provide useful data have been ongoing for several years. The article entitled "Rock Lithology and Porosity Determination from Shear Compressional Wave Velocity" by S. Domenico and published in 1984 in the journal Geophysics vol. 49, no. 8, and the article entitled "A Case Study of Stratigraphic Interpretation using Shear Compressional Seismic Data" by M. McCormick et al published in 1984 in the journal Geophysics, vol. 49, no. 5, focused on the V.sub.p /V.sub.s ratio as a useful diagnostic for lithology and porosity. However, in many situations, V.sub.p and V.sub.s are highly correlated. Thus, to extract useful information about perspective formations from shear data, it is necessary to measure relatively small changes in V.sub.p /V.sub.s in relatively thin intervals. Such techniques have proven difficult to accomplish with current shear wave technology.
However, there is one situation where V.sub.s behaves quite differently than V.sub.p. Such a situation occurs in those parts of the earth's subsurface where the rock properties are strongly influenced by the presence of gas in the pore space. When the pore fluid in a porous rock changes from brine to gas moving updip along a trap, V.sub.p generally decreases. In high-porosity, weakly-consolidated rocks, the decrease in V.sub.p is large and gives rise to observable, lateral changes in reflectivity and hydrocarbon indicators. In contrast, the pore fluid change causes only a small change in V.sub.s, resulting in negligible lateral changes in reflectivity. Thus, while a hydrocarbon indicator should stand out on the compressional wave seismic section as an amplitude anomaly, there should be little, if any, amplitude anomaly on the corresponding shear wave seismic section. If the compressional wave amplitude anomaly is caused by an anomalous lithology such as salt or coal, it should be equally apparent on the shear wave section, Thus, the best potential application of the shear wave should be distinguishing between true (hydrocarbon) and false (lithologic) amplitude anomalies.
One problem, however, with utilizing such comparative analysis methods is that the compressional and shear wave seismic sections must pertain, without ambiguity, to the same reflection point. Such a problem is solved by the pseudoshear method. Another problem associated with such method is that while a hydrocarbon indicator should produce an amplitude anomaly on the shear wave seismic section, limitations on the dynamic range of displays of such seismic data make it difficult to assess amplitude differences between compressional and shear wave seismic sections. Such a result is particularly common when the hydrocarbon formations producing amplitude anomalies are gas deposits in young, poorly consolidated formations where amplitude differences between compressional and shear wave data are difficult to assess.
An additional problem which is associated with methods for estimating shear wave reflection data from compressional wave amplitude offset variations is that such estimates, hereafter referred to as pseudoshear estimates, are extremely sensitive to small errors in NMO velocity Pseudoshear values are derived by measuring amplitude behavior of the P-wave data at each time horizon in an NMO corrected CDP gather. Correcting for time or velocity so that nearly perfect time alignments are produced from trace to trace for every reflector in the CDP gather is required to prevent the introduction of velocity errors. However, regardless of the care taken in determining stacking velocities when performing NMO corrections, typical NMO correction methods tend to leave some residual moveout in the seismic data which introduces errors in pseudoshear estimates later produced.